(a) Technical Field
The present invention relates to a stack for simulating a cell voltage reversal behavior in a fuel cell. More particularly, the present invention relates to a stack for simulating a cell voltage reversal behavior in a fuel cell, which simulates a voltage reversal behavior locally generated only in a portion of a plurality of cells of a fuel cell stack.
(b) Background Art
Polymer Electrolyte Membrane Fuel Cell (PEMFC) technology has been widely used as a fuel cell for vehicles. In order for the PEMFC to properly effectuate high power performance of at least tens of kW under various operation conditions of vehicles, the PEMFC needs to be stably operated within a wide current density range.
As is well-known, fuel cells are complied as a stack in which unit cells are stacked to meet power requirements therefrom. A Membrane-Electrode Assembly (MEA) is located at the innermost portion of the unit cell of the fuel cell stack. The MEA includes a solid polymer electrolyte membrane that can move hydrogen ions, and an anode and a cathode that are catalyst electrodes configured by coating a catalyst on both surfaces of the polymer electrolyte membrane. Additionally, a Gas Diffusion Layer (GDL) and a gasket are disposed outside MEA, i.e., outside the anode and the cathode. Also, a separator or bipolar plate is disposed outside the GDL to provide a flow field for supplying reactant gases and exhausting water generated from a reaction.
In a reaction for generating electricity in a fuel cell, after hydrogen supplied to the anode at which oxidation occurs in the fuel cell is divided into hydrogen ions and electrons, hydrogen ions move toward the cathode at which reduction occurs through a polymer electrolyte membrane, and electrons move toward the cathode through an external circuit. Also, in the cathode, oxygen molecules, hydrogen ions, and electrons react with each other to generate electricity, heat and water as a by-product.
If the amount of water generated from the electrochemical reaction in the fuel cell is appropriate, the generated water may serve to maintain an appropriate humidity from the MEA operate efficiently. However, if water is excessively generated, and the excessive water is not removed at a high current density, flooding may occur. This flooding may prohibit reactant gases from being efficiently supplied to the fuel cell, thus further deepening a voltage loss.
Due to various causes such as the flooding in the fuel cell, freezing during winter, and the abnormality of a reactant gas supply device, deficiency of reactant gases, i.e., hydrogen of the anode and oxygen or air of the cathode, that are used in the PEMFC may occur. Particularly, it is known that the hydrogen fuel starvation of the anode has a significantly detrimental influence on the performance of the fuel cell since it significantly reduces the cell voltage.
Generally, the hydrogen fuel starvation can be classified into overall hydrogen starvation in which the hydrogen supply is deficient throughout the entire fuel cell and local hydrogen starvation in which the hydrogen supply is locally deficient due to uneven distribution in spite of the sufficient hydrogen supply. Hydrogen fuel starvation frequently occurs under an operation condition such as uneven supply and distribution of hydrogen gas, a sudden increase of a fuel cell load demand, and/or fuel cell start-up. The overall hydrogen starvation can be relatively easily detected by monitoring the hydrogen supply using a sensor at a fuel cell system but the local hydrogen starvation in a portion of cells can be detected only by precisely monitoring each cell of the fuel cell stack individually using a stack voltage monitoring apparatus, requiring much more endeavor and a more complex control system. Here, a stack refers to a fuel cell including two or more cells, and the portion of cells refers to one or more cells that are equal to or less than 50% of the total number of cells in the stack.
FIG. 1 is a view illustrating a rapid drop of a cell voltage generated in a stack of an actual fuel cell vehicle. In the fuel cell stack shown in FIG. 1, the voltage of one cell rapidly drops to 0.1 V during operation for five minutes or more. When the rapid drop of the cell voltage occurs, the operation of the stack in a vehicle needs to be shut down so that the entire fuel cell does not become damaged, and then the abnormal operating cell needs to be replaced or repaired. This phenomenon usually occurs due to local hydrogen starvation. When a user continues to drive a vehicle while leaving the voltage dropped cell unrepaired, the vehicle may quickly reach a cell voltage reversal state in which the voltage becomes less than 0 V, accelerating the corrosion of carbon that is a catalyst support of MEA.
Generally, carbon widely used in the MEA catalyst support is thermodynamically unstable under an operation condition of PEMFC, and may be oxidized as the following chemical reaction formula:C+2H2O→CO2+4H++4e−(0.207 V vs. RHE)C+H2O→CO+2H++2e−(0.518 V vs. RHE)
Here, RHE means a reference hydrogen electrode. Generally, the above oxidation reactions slowly proceed, but can quickly proceed under a high voltage condition of a fuel cell. The high voltage condition is frequently caused by the hydrogen starvation or start-up/shut-down of a fuel cell vehicle. Also, when cell voltage reversal continues and then reaches an excessive voltage reversal state of about −2 V, the generated heat of the fuel cell becomes excessive, damaging the MEA and the gas diffusion layer as a whole, and particularly, causing a severe situation where a pin-hole occurs in the MEA and cells are electrically shorted. Thus, the fuel cell reaches a cell failure state in which the fuel cell cannot normally operate anymore and must be completely replaced.
Therefore, it is important to allow a fuel cell vehicle to stably operate by appropriately controlling the fuel cell vehicle before voltage reversal occurs and to develop fuel cell parts and systems with excellent durability which can withstand the voltage reversal. However, since cell voltage reversal due to local hydrogen starvation described above is difficult to detect before a fuel cell vehicle is being operated, a standard diagnosis technique that can reproducibly simulate this phenomenon should be secured before the vehicle begin operation.
As a typical method widely used to simulate the cell voltage reversal, there are methods that supply nitrogen instead of hydrogen to the anode or reduce the total hydrogen supply by reducing the Stoichiometric Ratio (S.R.) of the anode. However, in these methods, it is very difficult to reproducibly simulate the rapid drop of the cell performance and the voltage reversal due to the local hydrogen starvation of some cells out of several hundreds of cells in a stack of an actual fuel cell vehicle.
Accordingly, since there is no appropriate method for simulating the local cell voltage reversal in a fuel cell, it has been difficult to develop fuel cell vehicle control methods or fuel cell parts with excellent durability regarding the voltage reversal.
Furthermore, a typical method of simulating a cell voltage reversal in a fuel cell can use either a single cell or a stack including two or more cells. For example, FIG. 2 illustrates a conventional single cell 4 for a cell voltage reversal simulation in a fuel cell, where a cell 3 is disposed between both end plates 1 and 2. The above voltage reversal simulation method is for simulating a severe condition in which hydrogen deficiency is significant or hydrogen supply has completely stopped. Voltage reversal is induced by operating a fuel cell by supplying nitrogen instead of stopping hydrogen supply to the anode of cells of a fuel cell. This typical simulation method can simulate a voltage reversal generated over an entire fuel cell stack under a very severe condition in which hydrogen supply to the cells has ceased. However, it is very difficult to simulate an occurrence of a local voltage reversal in which the voltage rapidly drops only in some cells among hundreds of cells like a stack of a fuel cell vehicle. Also, the typical simulation method can be reportedly operated only in a low current density range, e.g., from 40 mA/cm2 to 200 mA/cm2 after nitrogen instead of hydrogen is supplied to the anode.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.